Seiji Arakaki

Short-Term Dynamics of Tidepool Fish Community: Diel and Seasonal Variation

Short-term temporal dynamics in a tidepool fish community were quantitatively examined through a series of direct observation at consecutive low tides in a temperate-subtropical intertidal of southwestern Japan. The visual observation was sufficiently effective compared with the capture by anaesthetics and allowed a continual, undisturbed monitoring. Species composition markedly varied between consecutive day versus night in spring-autumn, but not in winter. In contrast, species composition on consecutive days and on consecutive nights was relatively stable all through the year. The pattern of diel change in abundance varied among species and seasons. C. gulosus, Istiblennius sp. and Girella spp. showed the tendency of night-time occurrence, while B. fuscus individuals tended to occur by day. These trends of day–night differences were mainly observed in spring and summer and rarely in winter. Size differences between day and night were detected in B. fuscus, C. gulosus, Istiblennius sp. and Girella spp. This tidepool fish community demonstrated variation in abundance and body size on short time scales, which may be attributable to behavioural differences among species.

Habitat differentiation in the early life stages of simultaneously mass-spawning corals

The settlement process of coral larvae following simultaneous mass-spawning remains poorly understood, particularly in terms of population and community parameters. Here, the larval settlement patterns of Acropora corals, which are the most diverse genera of scleractinian corals at the species (haplotype) level, were investigated within a single subtropical reef. Across a 4-year period (2007–2010), the mitochondrial and nuclear molecular markers of 1,073 larval settlers were analyzed. Of the 11 dominant haplotypes of recruited populations, nine exhibited non-random patterns of settlement distribution. This result suggests that the actual habitat segregation starts during the early swimming larval stages of their life history, rather than by natural selection after random settlement. In addition, the presence of a depth-related settlement pattern supports that species-specific vertical zonation of coral larvae may play a role in the establishment of habitat segregation. Moreover, in some species that showed a preference toward the shoreward area of the bay, the settlement pattern was consistent with that of the adult distribution. This result indicates that the gametes were not mixed between fore and back reefs in the period from fertilization to settlement during the mass-spawning event, even within a single small reef. Another compatible hypothesis of this pattern is that the larvae are able to recognize various types of environmental information, facilitating the selection of optimal micro-habitats. Overall, Acropora coral larvae that are produced from a simultaneous mass-spawning event may have adapted to complex reef topography by means of multi-step habitat selection at settlement, corresponding to different spatial scales.

Microhabitat selection in intertidal gobiid fishes: Species- and size-associated variation

Patterns of space use and the individual-based behaviour of microhabitat selection were investigated in three intertidal gobiid fishes, Bathygobius fuscus, Chaenogobius annularis and C. gulosus, from Kyushu, southern Japan. While the three species tended to occupy slightly different types of tidepool, their patterns of distribution largely overlapped in the field. Laboratory experiments involving choice of shelter (i.e. underneath a stone plate) and four different substrate types were conducted to examine size- and time-related variation in habitat selection. The shelter area was preferred by small- and large-sized C. gulosus (day and night), large C. annularis (day and night) and small C. annularis (daytime only), while no preference was evident in small B. fuscus (day and night) and small C. annularis (night). Patterns of substrate choice also differed among species, size groups and between day and night. Size differences in substrate use were evident in B. fuscus and C. gulosus but not in C. annularis, while diel differences were shown by all species groups except large B. fuscus. The gravel and sand substrates tended to be used more frequently than the bare rock substrate, but the strength of preference of a particular substrate type varied among individuals/species. Our results demonstrate that habitat selection by the three gobiid species is variable depending on species, body size and time of day, which must ultimately bear upon mitigating intra-/interspecific interactions in tidepool environments.

Consuming Diversity: Analysis of Seasonal Catch Patterns in Multispecies Artisanal Reef Fisheries in North Sulawesi, Eastern Indonesia

Abstract: Despite the socioeconomic as well as ecological importance of smallscale fisheries in developing countries, there is a dearth of information on the state of artisanal fisheries in different regions of the tropical Indo-Pacific. In this study, catch patterns in small-scale artisanal fisheries within an area of high marine biodiversity in the western Pacific were analyzed using data gathered directly from the main fish market in Manado, North Sulawesi, eastern Indonesia. Of a total of 350 species identified among harvested fishes, the majority of species (ca. 90%) were closely associated with shallow reef habitats (<50 m), and open/deep-water species constituted a small proportion. There was a clear preponderance of relatively small (<50 cm) fish species among marketed fishes, with a steep decline in abundance of larger species, suggesting the possibility of overfishing. Faunal complementarity or distinctness between wet (November—March) and dry (April—October) season was lower for reef-associated fishes than for nonreef ones, reflecting the less-targeted nature of reef fisheries. Although relative catch patterns were broadly similar between wet and dry season, variability in catches as expressed by the variance of the truncated lognormal model was smaller in reef-associated than in nonreef species, indicating existence of year-round, relatively stable fishery activity centered on shallow reef environments. This finding points to the importance of reef habitats and their associated fish faunas for the artisanal fisheries of the tropical western Pacific.

Sex change and spatial distribution pattern in an intertidal holothurian Polycheira rufescens in the reproductive season

The patterns of sex change and spatial distribution in an intertidal holothurian, Polycheira rufescens (Chiridotidae; Echinodermata), were investigated on a stony beach in Amakusa, western Kyushu. Field caging experiments revealed that some individuals of P. rufescens underwent a sequential sex change from male to female via hermaphrodite stages and back to male again within a single reproductive season. The sex ratio of the population gradually changed from male dominance at first to equal proportions of males and females as the reproductive season progressed. Toward the end of the reproductive season, immature or spent individuals increased in proportion. Stone size appeared to be an important factor affecting the occurrence of P. rufescens individuals on a stony beach. Analysis of spatial distribution by means of Morisitas index of dispersion and nearest neighbor distances indicated that (i) males showed a slightly stronger tendency to aggregate than females, while females had a tendency toward uniform distribution; (ii) females tended to attract males, as shown by relatively short female-to-male distances; and (iii) there was a substantial variation in male-to-female distances, such that some males were positioned close to females while others were not. It may be suggested that spatial distribution of P. rufescens individuals during the reproductive period is partly dictated by the differential needs of individuals of different sexual states.